1. Field of Invention
This invention pertains to an implantable cardiac device such as a defibrillator wherein a high voltage pulses are applied to a patient to provide antitachycardia therapy. More particularly, the present invention pertains to an ICD having an automatic current limiting circuit.
2. Description of the Prior Art
Patients suffering from tachycardia (a higher than normal heart rhythm) are frequently treated with a defibrillator which is adapted to provide appropriate therapy. Typically this therapy consists of high voltage (relatively high energy content, shocks applied to the cardiac tissues. Frequently, the defibrillator is provided as an implantable cardiac device (ICD) installed surgically into the patient, typically at a site remote from the heart itself. Electrodes are also provided which have tips secured to externally to the heart, or internally for endo leads with leads connecting the electrodes to the defibrillator. The energy for the defibrillation shocks derived by the defibrillator is provided by one or more capacitors which, prior therapy, are charged to a nominal voltage and then discharged during shocks.
A problem with defibrillators is that because the impedance of the heart tissues through which the shocks are discharged is unknown and hence, it is difficult to control the current delivered during the shocks. Abnormally high current levels are undesirable because a high current may damage the heart tissues. It has been proposed, for example in U.S. Pat. Nos. 5,433,732 and 4,745,923 that current limiters be provided in series with one of the electrodes to thereby limit the maximum current that is delivered to the heart tissues during antitachycardia therapy. However, this approach is effective only if shocks of a single phase are applied. The approach is not effective for multi-polar-phase shocks. Moreover, the circuitry provided for the limiters is fairly complicated and uses a large number of components.